Load sensitive rotating anode x-ray tube speed selector



D. E. GRAVES 3,205,360

LOAD SENSITIVE ROTATING ANODE X-RAY TUBE SPEED SELECTOR Sept. 7, 1965 Filed Jan. l5, 1965 United States Patent O 3,205,360 LQAD SENSITIVEl ROTATING ANODE X-RAY TUBE SPEED SELECTOR Donald E. Graves, Wauwatosa, Wis., assigner to General Electric Company, a corporation of' New York Filed Jan. 1963, Ser. No. 251,589 4 Claims. (Cl. Z50-93) This invention pertains to means for protecting a multispeed rotating anode X-ray tube against operation under overload conditions and to means for automatically selecting the most desirable rotational speed for the current, voltage and exposure time factors at which the tube is set to operate.

It is known that the tube factors or energy input to an X-ray tube may be increased if heat dissipation at the focal spot `on the anode target is augmented by rotating the anode at high speed. For many years rotating anode motors that operated at 60 cycles, resulting in speeds of about 3,600 r.p.rn., were used. With the higher tube loadings that are now demanded, however, 180 cycle operation, resulting in rotational speeds of about 10,00() r.p.rn., have been adopted in order to avoid melting the target in the focal spot region. At the lower speeds, the duty on the bearings is severe because of the high temperature conditions under which they operate. At higher speeds bearing and tube life are reduced.

To conserve the X-ray tube, high speed rotation is used when the tube factor settings would exceed the safe limits for operation at the lower speed. This requires the operator to refer to a tube rating chart and exercise his judgment as to Whether the desired combination of voltage, current and exposure time necessitates high or low speed rotation. Erroneously choosing the lower speed may destroy the target surface, and wrongly choosing the high speed may reduce bearing and tube life.

An object of the present invention is to facilitate obtaining optimum life from an X-ray tube by eliminating operator judgment through the provision of means for sensing when the chosen tube factors would result in an overload at the lower speed and for automatically transferring to the higher speed only when necessary. An adjunct of this object is the provision of means for causing the protective device to reset in a condition that will prepare the tube for operation initially at the lower speed and which remains in that condition unless an overload for the lower speed is set again by the operator.

Stated in another way, it is an object of this invention to use low anode rotational speeds to the maximum degree and high speed rotation only when necessary, without involving human judgment.

Achievement of the foregoing and other more specific objects will appear from time to time throughout the course of the ensuing specification.

In general terms, an illustrative embodiment of the invention employs a relay switch means in the form of a thyratron which conducts and disables the circuit if the tube factors that are set will result in an overload on the X-ray tube. There are produced individual bias voltages that are respectively proportional to the prospective X-ray tube kilovoltage, current and exposure time settings. The effect of these settings is integrated by superposing the bias voltages on the grid of the thyratron which fires and locks out the circuit if there is an unsafe integration or combination of tube factors. The aforegoing features are described in D. E. Graves et al. Patent No. 2,745,020, dated May 8, 1956, and assigned to therassignee of this invention. The patent is not concerned with selecting proper rotational speed for the X-ray tube anode.

In the present invention, if the proportional bias 50 for effecting rotation of the anode.

voltages incident to the combined elect of the factor settings is indicative of an overload for low speed rotation, a new set of bias resistors is introduced so that the thyratron will operate at a higher level and at the same time the rotor speed is increased to accommodate the higher loading. If an overload setting exists after the thyratron samples both the low and high speed ratings, the exposure start circuit is held open until the X-ray controls are set to a safe level.

One preferred, but not exclusive, form of the invention will now be specifically described in conjunction with the drawing in which:

The sole ligure of the drawing is a schematic representation of an X-ray tube control circuit that includes the present invention.

Refer now to the drawing where a rotating anode X- ray tube may be seen. The cathode emitter of the tube is supplied through an isolating transformer 41 whose primary leads are connected to an auto-transformer 42. One conductor from the primary of transformer 41 includes a resistor 43 adjacent the autotransformer and to which connection is made with the autotransformer by a movable contact arm 44. Adjustment of arm 44 varies the input to the cathode of the X-ray tube, and accordingly, establishes the tube current, usually expressed in milliamperes (ma.).

The autotransformer 42 also supplies the primary of a high voltage step-up X-ray transformer 65 through an adjustable arm 45 which may be variously positioned on taps of the autotransformer. In one lead of the high voltage transformer primary winding, there is an X-ray contactor 46 which when closed, energizes transformer 65 and effects an X-ray exposure.

The secondary winding of transformer 65 supplies a high voltage rectifier bridge 47. The D.C. output leads from rectifier 47 are connected to the cathode and rotating anode of X-ray tube 40 and provide the voltage across the tube for accelerating electrons emitted from the cathode to the anode for production `of X-rays. Thus, the position of arm 45 on autotransformer 42 determines the kilovoltage factor at which an X-ray exposure may be made.

The third tube factor, that is, the exposure time interval, may be set by the operator turning a knob 48 on an exposure timer 49. Timer 49 governs the interval during which X-ray exposure contactor 46 will remain closed for taking of a radiograph or other technic.

Adjacent the anode of rotating anode tube 40 may be seen a representation of the stator field windings 49 and It will be described later how 60 cycle power may be impressed on the windings to rotate the anode at about 3,600 r.p.rn. and how cycle power may be impressed to cause rotation at around 9,000 or 10,000 r.p.rn. Of course, where different power line frequencies prevail, the rotational speeds maybe submultiples or multiples of the speeds mentioned for illustration. If desired, several power supplies having incremental frequency differences, or only one supply adapted for essentially stepless frequency changes, may be used.

In the ensuing discussion, frequent reference to relays and their operating contacts will be made. It would profit the reader to observe that all relays bear reference numerals below 20 and that their operating coils are designated by capital letters and a number, such as RE16. Contacts operated by the relays are designated by a similar number to which has been added a letter, such as 16A.

How the tube current, kilovoltage and conduction time intervals may be set has already been described. The discussion will continue with a description of how a bias voltage, that is proportional to each of these tube factors,

may be produced for controlling an overload-responsive thyratron switch 51 or other switch relay means. There is impressed on the control grid of thyratron 51 a D.C. bias voltage that is non-linearly proportional t the time interval setting. Also impressed on the control grid is an A.C. 'bias which is proportional to the combined `effects yof the kilovoltage and current settings of the X-ray tube factors.

For the purpose of supplying the time interval dependent, D.C. bias, there is provided a voltage stable rectifier 52 which is supplied with A.C. from a transformer 53 whose primary is energized from .appropriate taps on autotransformer 42. Rectifier 52 has two D C. output terminals `d and 55. Terminal 54 supplies a Voltage divider including an adjustable resi-Stor 56, a potentiometer resistor 57 and fixed resistors SS, -all of which are in series and return to D.C. supply terminal 55. The D C. bias for thyratron 51 is taken off the potentiometer resistor 57 with an adjustable switch arm 59 that is directly connected to the cathode of thyratron S1. The control grid side of the D.C. bias originates at terminal 5S and passes through one in `an array of resistors, which may be disregarded for the moment, and appears on one of terminals of a multi point selector switch 60. Connection is made to the control grid by an adjustable arm 61.

Potentiometer arm 59 in the voltage divider is coupled with control knob 4S on exposure timer 49 as symbolized by the broken line 62 emanating from each of them. It should be noted that potentiometer resistor S7 is of the non-linear variety. That is, the voltage which is taken from it by way of potentiometer arm 59 varie-s essentially logarithmically with the position of the latter. This means that the D. C. bias on the thyratron is affected logarithmically by the exposure time setting. The purpose of this is to make the D.C. bias conform to the configuration of the curves on the tube rating charts. For any given tube current, the permissible applied X-ray tube voltage varies logarithmically with respect to time.

Also concurrently applied to the control grid of thyratron 51 is an A.C. bias potential that is proportional to the X-ray tube current and kilovoltage factor settings. This is accomplished by an array of resistors and contacts shown at the right side of the drawing. There are two vertical columns of resistors 70 and 71, respectively. One of the resistors in column 70 is the A.C. bias circuit at any time that the X-ray tube rotor is operated at 60 cycles or in the 3,600 r.p.m. range. Each resistor, such as R72 corresponds with a distinctive X-ray technic or current setting. For instance, the top resistor R72 may be in circuit yat a 100 ma. tube current setting, while the lowermost resistor R76 may be in circuit when the tube current is set for 500 ma. Associated with the resistors in column 70 are individual contacts 12F to 12J which are shown normally closed as they would be when the tube factors are set so that 60 cycle operation of the rotating anode would be permissible.

Column 71 comprises potentiometer resistors R77 t0 R81 an-d these two have associated contacts 12A to 12E, respectively. One of the resistors such as R77 is in the A.C. bias circuit only after the circuitry has determined that unsafe conditions exist for operating the tube at 60 cycles and that it is desirable to transfer it to the higher rotational speed corresponding with 18() cycles in this illustration.

It will be seen that intermediate the contacts in the columns beginning with 12A and 12F, respectively, there is a tap which runs to the various points on selector switch 60 so that the connection can be made from one of them to the thyratron grid through selector arm 61. It will be noted that arm 61 is coupled with X-ray tube current control arm 44 adjacent autotransformer 42 as symbolized by the broken line 82. Hence, an A.C. bias that is partially dependent upon the X-ray tube current is automatically chosen when the X-ray tube current factor is selected. v

The A.C. bias is also dependent upon the chosen X-ray tube kilovoltage. For this purpose the columns 7i) and 71 Iof bias resistors are supplied with A.C. from the secondary of a transformer S2 whose primary is connected to the same adjustable tap 4S on the autotransformer which enables supplying an adjustable voltage to the primary of high voltage X-ray tube transformer 65. Hence, when a new tube kilovoltage is selected by positioning arm on the autotransf-ormer, a corresponding and proportional A.C. voltage is applied to both columns of resistors and 71.

The operating mode of the circuitry is such that when the -desired tube factors are set, the protective circuit will determine whether the exposure can safely be made at the lower rotational speed yor whether a transfer to higher rotational -speed is necessary. If at the higher speed an overload still exists, the device will again prohibit making an X-ray exposure unless the tube factors are reset. Thyratron 51 senses if a transfer is necessary or an overload exists after transfer. A further explanation of what happens elsewhere in the circuitry during these events will be explained shortly hereinafter.

It will be `observed that the controls are .adjusted for one voltage, -current and time combination. By virtue of the position of arm 61 in selector switch 6i) it follows that the A.C. bias voltage derived from R72 would be applied to the thyratron by reason of contact 12F being closed. If the thyratron senses an overload at 60 cycle operation, all of the contacts 12F-J open and the contacts 12A-E close to bring about conditions for cycle operation by virtue of RE12 lbeing energized. The same sequence of events takes place if any of the other 60 cycle current, time and voltage settings are exceeded in accordance with whether R72 or other resistors up to R76 are in circuit.

Whenever an X-ray exposure is contemplated, it is necessary to bring the X-ray tube rotor up to speed before an exposure can be initiated. Consistent with the X-ray tube conserving features of the instant invention, the tube is first always brought up to 60 cycle rotational speed and only raised to the higher speed at 180 cycles if the tube factor settings call for it. The rotor is controlled by a hand switch assembly that includes a rotor switch S1, seen in the upper left portion of the drawing. When S1 is closed it energizes REI() which is in series with it and across 0 and 115 volt lines taken from autotransformer 42. REN, when energized, closes its contact 10A so that 60 cycle power is supplied directly to the windings 49 and 50 of the anode motor through normally closed contacts 18A and 18B. REN) also closes its 10B contact which energizes a rotor time delay device 82. After two and one-half seconds elapse, the time delay device energizes RE17 which in turn closes its contact 17A. If safe conditions exist, then pressing -of switch SZ will permit contacter 46 to close and make an X-ray exposure. Operation of rotor control relay REM) also closes its contacts 10C, which is in the thyratron 51 plate circuit, and 10D which is near the top of the drawing in circuit with RES.

It will be observed that the plate circuit of thyratron 51 has a relay RE16 in it. The plate circuit is on A.C. and supplied from the same transformer 53 which supplies the rectifier and uses as 1a common return ff-rom the cathode the same ci-rcuit through which .the D.-C. bias is impressed. That is, lthe cathode return is through potentiometer arm 59, resistors 57 and 58, and back to D.C. terminal 55 which also serves as -a common A.C. terminal.

If the X-ray tube factor settings are such that thyratron 51 senses an overload at 60 cycle operation, REIG is energized. 'It immediately opens its contact ,16A which is in series with exposure switch S2, thus preventing an expolsure from being made. When 16A opens, a warning light 83 goes on momentarily to indicate that an unsafe setting has been selected.

If the operator continues to hold rotor switch S1 closed so as to maintain operation of the rotor, la transfer to 180 cycle high speed rotation will be effected. This results from the lfact that .when overload relay RE16 operated, it also closed its contact 16B which permitted energization of Ia relay RES through contact 16B initially. RES Aand its associated contacts are supplied from the secondary of transformer 53, which secondary also supplies the filaments of the electron tubes. When RES -is energized through 16B the former closes its 8A contact and locks itself in, which is possible since contact 10D has closed.

Furthermore, when `RE8 is energized it Ialso closes its "8B contact which permits supplying A.C. power to energize a 180 cycle power supply l8S for the X-ray tube roto-r circuit. -It is desirable that the 180 cycle p-ower supply Ibe a solid state inverter that requires :little warmup time before it is ready yto provide power to the rotor.

It will be seen that 60 cycle power is supplied from lines 86 to the rotor windings through normally closed contacts 18A and y18B. When 180 cycle power supply S5 is turned on however, due to energization of RES, the latter closes its 8C contact for the purpose of energizing a relay REIS and following a second time delay, relay RE18 is energized. This connects the rotor to 180 cycle power supply `85 over lines 87 feeding through contacts 18C :and 18D at the end of the 3A second time delay.

When RES is energized on overload, its contact 8D closes and energizes RE12 which is located in the drawing `ab-ove its associated contacts 12A-E .and 12F-J. Upon energization of RE1'2, the series of contacts '12F-I open and 12A-E series close so as to permit a new A.C. bias level to be imposed on the grid of thyratron 51 concurrently with the change to higher rotational speed. This inserts o-ne lof the A.C. bias circuit potentiometer resistors in the column 71 in series with selector switch 60.

If the combination of tube current, Ikilovoltage and exposure time intervals are such as to produce a bias voltage that allows thyratron 51 to conduct event at the higher 80 cycle anode rotational speed, RE16 is again energized. This -opens contact l16A and likewise -opens the exposure circuit so `that pressing exposure Ibutt-on S2 will be to no avail. Overload indicator lamp 83 then lights and stays Again, when the rotor button is released, the circuit is cleared and restored to preparation 'for 60 cycle operation by virtue of rotor -contact `10C opening to thereby deenergize overload relay yRE16 and to open its associated contact 16B `along with 10D which latter two contacts deenergize RES when they open. Any time the rotor button is open the circuit is restored to conditions which operate the X-ray tube anode motor at 60 cycles or the lower rotational speed.

In summary, it can Ibe seen how the operator can choose current, voltage and time interval settings which he thinks he needs to perfo-rm the desired technic without regard 4for whether it is most desirable to operate the X-ray tube anode motor -at its high or llow speed. If conditions are safe for operation lat a low speed .then the rotor will operate at low speed. One cannot operate at the higher rotor speed unless the safe setting limits for 60 .cycle operation `are exceeded. Moreover, when they are exceeded, high speed rotation is automatically selected. If overload conditions still prevail, making an exposure is prohibited, and on the next attempt, after the tube ,factors are -re-selected, the Isensor will again determine whether an exposure can be made at the lower rotational speed before it effects la transfer, after which the same sensor `will determine if there is still lan `overload and prevent energization of the X-ray tube if there is.

Although one lpractical embodiment of the invention has been described in considerable detail, .such description is intended to be illustrative rather than limiting, for the principles of the invention may be variously embodied 6 and the Iscope of the invention is to be determined only by interpretation of the claims which follow:

It is claimed:

1. A control system for a multi-speed rotating anode X-ray tube comprising:

(a) an X-ray tube having an anode rotor and field windings for driving the rotor,

(b) relatively low and higher frequency power supplies in circuit with the field windings for driving the rotor at relatively low `and higher rotational speeds,

(c) an electronic switch including a control element which is adapted to render the switch conductive at a predetermined bias level,

(d) adjustable sources of power for the X-ray tube and an adjustable conduction interval timer therefor, (e) means for applying a control bias on the element in accordance with adjustment of the power and time factors for the X-ray tube, said control bias having one value that corresponds with the lower rotational speed of the anode,

(f) said control bias means rendering said electronic switch conductive if the bias corresponds with an overload on the X-ray tube for the lower rotational speed of the anode,

(g) relay rneans energized by operation of said electronic switch to connect a higher frequency power supply to the field windings to thereby increase the rotor speed, and

(h) means operable concurrrently with said last named relay means to change the value of the bias and to operate the first switch means again if the signal corresponds with an overload on the X-ray tube for the chosen tube factors at a higher anode rotor speed.

2. The invention set forth in claim 1 including:

(a) means for resetting the relay means that are operated by the electronic switch,

(b) a rotor control switch by means of which the rotor is energized at all speeds of rotation and which when released operates said resetting means to prepare said rotor for operation at its lowest speed when said rotor control switch is operated again.

3. A control system for a multi-speed rotating anode X-ray tube comprising:

(a) an X-ray tube having an anode rotor and field windings for driving the rotor,

(b) low and higher frequency power supplies in circuit with the field windings for driving the rotor at relatively low and higher speeds,

(c) an electronic switch having a control element for a signal that is adapted to render the switch conductive at a predetermined signal value,

(d) means for developing a first set of signal bias values in relation to the current, kilovoltage and conduction time factors of the X-ray tube for its lower rotational speed,

(e) means for developing a second set of bias signal Values for the same X-ray tube factors for a higher rotational speed, and

(f) means for connecting the second set of bias signal developing means to said control element in response to said electronic switch being rendered conductive bya signal in the first set corresponding to an overload condition occurring in the X-ray tube whereby the high frequency power supply is connected to the field windings so as to rotate the anode rotor at the relatively higher speed.

4. A control system for effecting the lowest speed of rotation for a multispeed rotating anode X-ray tube without exceeding the safe thermal rating of the tube during a radiographic exposure, comprising:

(a) an X-ray tube having an anode rotor and field windings for driving the rotor,

(b) a selectable frequency power supply means in circuit With the field windings for driving the rotor at relatively low and higher rotational speeds corresponding with respectively lower and higher frequencies,

(c) an electronic switch having a control element for rendering the switch conductive when a bias of predetermined value corresponding to an unsafe condition for the X-ray tube is applied to the control element,

(d) a source of electrical power,

(e) a first relay means in circuit with the electronic switch and the electrical power source,

(f) means for setting the desired conduction time interval, the voltage and the current for the X-ray tube,

(g) a bias circuit including means to adjust the bias in accordance with the time, current and voltage settings to produce a bias value which will render the electronic switch conductive in response to an overload condition occurring in the X-ray tube to thereby operate said first relay means if the bias value equals the predetermined value for one rotational speed of the anode,

(h) a second relay means that is controlled by operation of the first relay means to select a higher frequency for the field windings to thereby increase rotational speed of the rotor when an overload condition occurs in the X-ray tube,

(i) a third relay means controlled by the second relay means to switch the bias circuit to supply a diiferent bias Value to the control element and thereby render the electronic switch conductive if the settings are unsafe for the higher rotational speed and non-conductive if the settings are safe,

(j) a manual radiographic exposure and rotor rotation initiating switch means,

(k) a switch contact that is in series with the exposure initiating switch and that is controlled by the first relay switch means to render the exposure initiating switch ineffective if an overload setting prevails at either rotational speed,

(l) the said manual switch means having a relay means in circuit therewith which resets the second relay means when operation of the manual switch is discontinued, to thereby cause the rotor to turn at its lower speed at the beginning of the next operational sequence.

References Cited by the Examiner UNITED STATES PATENTS 2,092,618 9/37 Bouwers 250-100 2,185,826 1/40 Atlee 250-93 2,222,549 11/40 Verhoeff 313-60 X 2,745,020 5/56 Graves et al. Z50-93 2,798,963 7/57 Saget 250-93 RALPH G. NILSON, Primary Examiner. 

1. A CONTROL SYSTEM FOR A MULTI-SPEED ROTATING ANODE X-RAY TUBE COMPRISING: (A) AN X-RAY TUBE HAVING AN ANODE ROTOR AND FIELD WINDINGS FOR DRIVING THE ROTOR, (B) RELATIVELY LOW AND HIGHER FREQUENCY POWER SUPPLIES IN CIRCUIT WITH THE FIELD WINDINGS FOR DRIVING THE ROTOR AT RELATIVELY LOW AND HIGHER ROTATIONAL SPEEDS, (C) AN ELECTRONIC SWITCH INCLUDING A CONTROL ELEMENT WHICH IS ADAPTED TO RENDER THE SWITCH CONDUCTIVE AT A PREDETERMINED BIAS LEVEL, (D) ADJUSTABLE SOURCES OF POWER FOR THE X-RAY TUBE AND AN ADJUSTABLE CONDUCTION INTERVAL TIMER THEREFOR, (E) MEANS FOR APPLYING A CONTROL BIAS ON THE ELEMENT IN ACCORDANCE WITH ADJUSTMENT OF THE POWER AND TIME FACTORS FOR THE X-RAY TUBE, SAID CONTROL BIAS HAVING ONE VALUE THAT CORRESPONDS WITH THE LOWER ROTATIONAL SPEED OF THE ANODE, (F) SAID CONTROL BIAS MEANS RENDERING SAID ELECTRONIC SWITCH CONDUCTIVE IF THE BIAS CORRESPONDS WITH AN OVERLOAD ON THE X-RAY TUBE FOR THE LOWER ROTOATIONAL SPEED OF THE ANODE, (G) RELAY MEANS ENERGIZED BY OPERATION OF SAID ELECTRONIC SWITCH TO CONNECT A HIGHER FREQUENCY POWER SUPPLY TO THE FIELD WINDINGS TO THEREBY INCREASE THE ROTOR SPEED, AND (H) MEANS OPERABLE CONCURRENTLY WITH SAID LAST NAMED RELAY MEANS TO CHANGE THE VALUE OF THE BIAS AND TO OPERATE THE FIRST SWITCH MEANS AGAIN IF THE SIGNAL CORRESPONDS WITH AN OVERLOAD ON THE X-RAY TUBE FOR THE CHOSEN TUBE FACTORS AT A HGHER ANODE ROTOR SPEED. 